In the oil and gas industry, wells are drilled deep into the earth's crust for the purpose of finding and retrieving petrochemicals. Operating companies, who own or manage such wells, as well as oilfield services companies, evaluate wells in a variety of ways, for example, by acquiring formation cores. These formation cores may be obtained using coring tools. These tools are conveyed on a drilling collar or wireline suspended into the well and are adapted to drill into a side-wall of a borehole to obtain formation samples, also known as cores.
The assessment of formation characteristics acquired from formation cores is often crucial to the decision-making process concerning development plans for petroleum wells that are being evaluated as part of an exploration or production activity. Depending on the results of the evaluation, the well could be drilled deeper, plugged and abandoned as non-productive or cased and tested. The evaluation may also be inconclusive and the determination made that additional evaluation such as, for example, further acquisition of cores of the formation, is required before a decision on the disposition of the well can be made. The results of the core analysis as interpreted from a well log may also help determine whether the well requires stimulation or special completion technologies such as, for example, gas lift or sand control. The decisions made from well evaluations are very difficult, often made with imperfect information, have huge economic impact, and frequently have to be made very quickly. Mistakes, or even mere delay, can be extremely expensive.
There are several different types of tools for obtaining side cores. One approach is to manipulate a rotating hollow cylindrical coring bit into the side-wall of the borehole. As the rotating coring bit is forced into the sidewall, a small sample of the formation, known herein as the core, is collected in the interior of the coring bit. An example of a side-coring tool is the Mechanical Side-Coring Tool (MSCT™) of Schlumberger Technology Corporation. Side-wall core samples are acquired by the MSCT™ using rotary drilling whereby no percussion damage is caused by rotary drilling into the side-wall of the borehole. The Mechanical Side Coring Tool is operable to acquire up to twenty side-wall core samples during a single trip into the borehole. The rotary drilling of the side-wall core by the MSCT™ preserves the properties of the side-wall core samples thereby allowing accurate measurements of parameters such as relative permeability and secondary porosity.
Production company personnel at a well site or other personnel involved in planning a logging job may plan for a side-wall coring job that involves acquiring side-wall cores for particular depths of interest. A coring tool is then lowered to the depth of interest and coring operations are performed at these depths. Core samples are collected in the tool and the entire apparatus retrieved to the surface. Upon retrieving the coring tool, these personnel may discover, to their dismay, that a fewer number of cores were actually acquired during the job than what was planned. An additional problem from the failure to acquire all planned side-wall cores is a difficulty in sorting out which side-wall core associates to a specific planned depth of interest. Furthermore, the lack of core analysis in current coring tools results in delay in testing and updating any reservoir model until such time the acquired side-wall cores are analyzed in the laboratory.
Electromagnetic induction and propagation logging tools are commonly used for determining electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, reasonably determine certain petrophysical properties of the formation.
Induction tools have been used for many years to measure the resistivity of earth formations surrounding a borehole. Induction logging tools measure the resistivity (or its inverse, conductivity) of the formation by inducing eddy currents in the formations in response to an AC transmitter signal. The eddy currents induce secondary magnetic fields that in turn induce voltages in receiver antennas. Because the magnitudes of the eddy currents depend on formation conductivities, the magnitudes of the received signals thus reflect the formation conductivities. A typical induction tool includes at least two induction arrays having different spacings between the transmitters and the receivers for different depths of investigation (DOI).